During development, cells must make choices between alternative pathways, and they must maintain appropriate developmental timing so that differentiation and morphogenesis are coordinated. This is accomplished by an assessment of the cell's environment through sensory signal transduction mechanisms and integration of the relevant extracellular and intracellular information to allow the appropriate decision to be made and timing to be maintained. Recent genetic evidence suggests that the major signaling mechanism of prokaryotes couples to a major eukaryotic effector to regulate the timing of multicellular development and choices between alternative developmental pathways and cell fates in Dictyostelium. We hypothesize that a two-component phosphorelay, controlled by sensory histidine kinases DHKC and DHKB, regulate these important aspects of development by modulating the activity of cAMP-dependent protein kinase. Molecular genetic and biochemical approaches are proposed that will establish the molecular mechanisms that result in the regulation and control of multicellular development. Specifically, experiments in vivo and in vitro will examine the biochemical details of the phosphorelay by characterizing the autophosphorylation, phosphatase, and phosphotransfer activities of the various components and by determining the effects of one component on the activities of the others. Ectopic expression of stimulus-independent activators or inhibitors of the pathway, using cell type-specific promoters, will reveal in which cell type(s) the pathway functions and if the pathway results in cell to cell signaling to couple differentiation and morphogenesis. Finally genetic screens for suppressers or synthetic phenotypes will identify new and perhaps novel components of the regulatory pathway, including upstream inputs, phosphorelay regulators, and downstream effectors.